A low brass instrument also referred to as a baritone. Comes from the word euphonic, meaning good sound. The single most awesome instrument in the world, if not simply for the name. Most people don't know what it is and ask in their peasantish ignorance. These people should be smacked.

The single most important instrument in the low brass section of a band. The players are often anal about the performance of other band members epically trumpets, and they are often the best players in the band.

Band Director: Thank you Euphonium's for an exceptional performance.
Euphonium player: It would have been better but the trumpets egos got in the way!

n. a large 3- or 4-valved brass instrument resembling a small tuba; it is similar to the baritone horn but, while the baritone has a cylindrical (straight) bore, the euphonium has a conical (grows larger as it progresses towards the bell) bore, warming up the sound

The euphonium has a dark, rich sound--except when too much garbage has been dropped down the bell.

Mosty definitely the most admired instrument in the band. The euphonium is a precious part of the symphonic band, as is the baritone to the marching band, because it provides a rich, dark sound to sustain the bottom of the band. This is the sounds that all other band members envy. These instruments, with the assistance of the rest of the low brass, give the band the edge needed to win superiors. Plain and simple, euphoniums rule!

a low brass instrument with 3 or 4 valves that can be played in treble or bass clef. hardly anybody knows what it is so euphonium players often get tired of explaining it. they are usually the least ignorant section in low brass and the smallest and most talented section in the band.

band director: can someone please explain the coda to the trombones!?
euphonium section: we can! we can!

A mysterious instruments played by the select few. It is the most important instrument of the low brass. The players get sick and tired of people asking what a Euphonium is, and most likely didn't tell you. That is why you are on Urban Dictionary.